Urinary System

Urinary System Functions

• Regulate BP through the kidneys. Fluid control leads to blood volume control

• Balance pH by ions and acid-base in the kidney

Fibrous/Renal Capsule

Compose of dense irregular connective tissue

Renal Cortex

• The outer region

• The renal columns is the extensions of the cortex that divide the medulla into renal pyramids

Renal Pelvis

From major calyces, urine drains into a single large cavity and out through the ureter

Nephron

Each kidney composes of 1 million microscopic functional filtration unit

Glomerulus

Thick tangle of capillary loops (glomerular capillaries) that blood enters

Bowman’s/Glomerular Capsule

Surrounds the glomerulus with two layers

• Visceral layer that is permeable

• Parietal layer that is impermeable

• Capsular space between the two layers where the filtrate goes

Proximal Convoluted Tubule

• First part of the renal tubule

• Most reabsorption occurs here

• Reside in the renal cortex

Nephron Loop: Loop of Henle (Function)

Purpose is to recover water and sodium chloride from urine

• Descending limb: Highly permeable to water but impermeable to solutes. Extends from proximal convoluted tubules

• Ascending limb: Highly permeable to sodium (Na⁺) and chloride (Cl⁻) but impermeable to water. Terminates at distal convoluted tubules

Distal Convoluted Tubule

• Ions reabsorption (Na, K, Ca), and pH balance

• Final portion of renal tubule that terminates at the collecting tubule

Collecting Tubules & Ducts

Finetune urine composition through reabsorption and secretion to maintain fluid, pH, and electrolyte balance

Filtrate & Urine Flow

• Blood flows through glomerulus, the water & solutes filter out of the plasma into Bowman’s capsule forming the filtrate

• Pass through PCT → nephron loop → DCT, the substance now called tubular fluid

• Enters smaller collecting tubules and empties into larger collecting ducts

• After leaving the ducts, it’s called urine and it travels through renal pelvis → ureter

Urine Formations (3 steps)

• Filtration: selectively form filtrate

• Tubular reabsorption: important remain substances are reabsorbed back into the blood

• Tubular secretion: Additional substances such as ions, drugs & toxins move out of the blood into tubular fluid for excretion

Filtrate

• Certain solutes move out of the blood into the capsule

• Freely filtered (waste) water, glucose, ions, amino acids, hormones

• Limited filtered small proteins

• Not filtered formed elements, and large proteins

Glomerular Hydrostatic BP

• The BP in glomerulus that pushes water and dissolved substances out of glomerulus into capsular space

• Opposed by

  • Capsular hydrostatic pressure: pressure that already in the capsule

  • Blood colloid osmotic pressure: pressure in blood due to solutes present

Glomerular Filtration Rate (GFR)

• Higher filtration rate results in more important substances being excrete as urine because there is less time to be reabsorbed into the blood

Intrinsic Control of GFR

• Renal autoregulation: maintain constant BP and GFR regardless of changes in systemic BP by altering the size of afferent arteriole as response

Tubular Reabsorption (Location)

• Occur in the PCT

• Extensive microvilli increase surface area

Proteins

If small/medium sized are filtered out, they are broken down into amino acids and then transport back into the blood

Ureters (Function)

Fibromuscular tubes that transfer urine from renal pelvis to bladder

Urinary Bladder

• Consist of rugae that can expand and shrink

• 3 layers of smooth muscle

Urethra (Function)

• Fibromuscular tube that conducts urine from bladder to the exterior of the body

Urethral Sphincters

Restrict release of urine until pressure in bladder is high enough

• Internal urethral sphincter: involuntary, smooth muscle

• External urethral sphincter: voluntary, skeletal muscle